1. Field of the Invention
The present invention relates to method and device for communicating data by use of a plurality of communication lines and, in particular, to such data communication method and device suitable for securing and determining a previously set number of communication lines.
2. Description of the Related Art
Conventionally, when data is communicated between communication equipments such as a facsimile or the like by use of a public phone lines network, generally, only a single line is used and data is transmitted by means of a serial transmission system. In such serial transmission system using a single line, when image data having a large quantity of information is transmitted, then the data communication time is quite long. In order to solve this problem, a system (a parallel transmission system) can be provided in which the data is divided into a plurality of blocks and then these block data are transmitted in parallel by use of a plurality of communication lines.
In recent years, there has been developed and gradually but widely used an integrated service digital network (which is hereinafter referred to as ISDN) which is able to transmit data using a plurality of communication lines simultaneously. If the ISDN is used, then the above-mentioned parallel transmission system can be executed with ease. Referring more particularly to the ISDN, it is a communication network which integrates the information of various kinds of communication equipments such as telephones, facsimiles, telexes and the like connected through a bus to a domestic service line terminal unit (DSU) in a digital form to thereby allow the integrated digital data to be communicated in parallel between the above-mentioned communication equipments by use of a plurality of communication lines.
Further, with the widespread use of the ISDN, there has been also studied a communication system in which data is divided into a plurality of block data and the block data are transmitted in parallel at the transmitting end, and the block data are restored to the original data thereof at the receiving end. This is hereinafter referred to as a super-high-speed communication system.
However, the above-mentioned conventional super-high-speed communication system has the following problems:
That is, in the conventional super-high-speed communication system, usable communication lines are previously registered and an appropriate number of communication lines are selected out of the registered ones for data transmission. The conventional system is convenient for the super-high-speed communication because a previously set number of communication lines are secured for the super-high-speed communication. However, since this means that the communication lines are occupied only for the super-high-speed communication, there is no room for use of the communication lines by normal communication.
In other words, when the normal communication interrupts the communication lines, it is not possible to recognize at either of transmitting or receiving end that the lines at the other end are being used by the normal communication. For this reason, even if the transmitting side tries to determine the number of communication lines arbitrarily for the super-high-speed communication, the desired number of communication lines cannot be secured. This is because a large number of communication lines must be always secured for the super-high-speed communication, and, more particularly, because there is no clear standard for determination of special communication lines necessary for the super-high-speed communication.